In a curtain coating apparatus, a moving receiving surface is coated by causing a free-falling curtain of coating composition to impinge on the receiving surface to form a layer thereon. One curtain coating apparatus is described in U.S. Pat. No. 3,508,947 to Hughes wherein a composite layer comprising a plurality of distinct layers is formed on a slide hopper and dropped therefrom to form a free-falling curtain. The edges of the curtain must be held to maintain the width of the curtain; otherwise, the curtain narrows as it falls as a consequence of surface tension. To counteract surface tension and maintain curtain width, the curtain is placed in wetting contact with a substantially vertical solid support surface called an edge guide.
In the manufacture of photographic materials comprising many layers, the coated uniformity of the individual layers and the composite layer is critical. Layer uniformity must generally be better than about 2%, and uniformity may have to be significantly better than that over small areas such as a 35 millimeter film frame or over large areas such as the length of a motion picture film.
One source of coating nonuniformity that may occur arises from stationary waves in the curtain. These waves originate at the lateral and top edges of the curtain and propagate laterally into the main body of the curtain. Along the boundary of a stationary wave, the wave speed of propagation of curtain disturbances is in balance with the speed of the falling curtain. Where the wave boundary meets the support being coated, a nonuniformity in the form of a longitudinal streak may be observed in the coated composite layer. More generally, the uniformity of the coated layers can be affected everywhere beneath the wave boundaries. Whether a streak or more diffuse nonuniformity is objectionable depends upon its severity and upon the release specifications for the product.
Standing waves become more common as more layers are coated simultaneously and as the flow rate of the composite layer increases. There is a trend to coat more layers simultaneously to reduce cost and to accommodate new products with additional layers to provide features. There is also a trend to higher coating speeds to reduce manufacturing costs. Therefore, preventing loss of product to objectionable coating nonuniformities from stationary waves is becoming increasingly difficult.
Waves may originate at the hopper lip because of mechanical damage or contamination. Another cause of waves is a discrepancy between curtain trajectory and the profile of the edge guide at the hopper lip. Curtains formed asymmetrically, such as by a slide hopper, do not depart from the lip vertically downward except at low flow rates. More commonly, the curtain abruptly bends backward toward the hopper body at the hopper lip. If the first portion of the edge guide does not accommodate this change of direction, then a stationary wave may arise at the juncture of the edge guide and the hopper lip.
Waves most commonly originate at edge guides because of mechanical damage or their geometric configuration. Solidification of coating composition on the guide from the congealing or drying of coating composition can initiate waves. Such buildup can be reduced or eliminated by flushing the edge guide with a suitable solvent such as water and controlling ambient conditions including temperature, vapor pressure, and air motions. The low viscosity of the solvent also reduces the drag of the edge guide on the free-falling curtain. Coating latitude is compromised at the edges of the curtain if the velocity there is significantly reduced compared to the main body of the curtain because of drag. Curtain attachment to the full length of the edge guide is also compromised.
Introducing a liquid for flushing and lubricating the edge guide without introducing waves has proven difficult. A poorly configured distribution body can induce waves. Because of the very low viscosity of an effective lubricating liquid, the flow issuing onto the edge guide can be turbulent. Turbulent flow can create traveling waves in the curtain because the turbulent disturbances exciting the waves are moving and transient. If the lubricating liquid jets into the main body of the curtain, waves can also arise, and additional nonuniformity can result as the jet physically displaces the composite layer from the edge guide.
U.S. Pat. No. 4,830,887 discloses a means for introducing lubricating liquid shown in FIGS. 1a and 1b. The edge guide is a narrow tube, and the liquid is supplied to the top of the tube. The liquid issues from a long, narrow slit in the tube that is in wetting contact with the edge of the curtain. Such a narrow tube is prone to turbulent flow and plugging. Furthermore, tubes have proven less resistant to the formation of stationary waves than dual wires.
U.S. Pat. No. 4,974,533 discloses a means for introducing lubricating liquid shown in FIG. 1c. The liquid issues vertically downward from a tube with an opening adjacent to the edge guide. The edge guide is the edge of a plate in wetting contact with the curtain. Such narrow tubes are prone to turbulent flow, and plates have proven less resistant to the development of stationary waves and other edge nonuniformities than dual wires.
Edge guides preferably comprise two thin, parallel, closely spaced wires lying in a plane perpendicular to the hopper lip as disclosed in U.S. Pat. No. 5,328,726. The wires are supported only at the top and bottom of the edge guide. Wire edge guides have proven generally superior for curtain coating and in particular have demonstrated resistance to forming standing waves. Damaged wire is cheaply replaced. Flushed wire edge guides economically and practically solve the problems of buildup and drag. For distributing lubricating liquid to the wires, U.S. Pat. No. 5,328,726 teaches the use of a conduit with an opening directed vertically downward that encompasses the wires as shown in FIG. 1d. Again, so small a conduit is prone to turbulent flow. Additionally, running wires through the conduit complicates their replacement. The configuration is also prone to initiating a stationary wave at the lowermost and innermost edge of the distribution body, as shown in FIG. 2.
U.S. Pat. No. 5,358,569 teaches the use of a gradually curved conduit encompassing the wires with an opening directed vertically downward as shown in FIG. 1e. This reference teaches how to size the conduit and how to restrict its curvature to ensure laminar (i.e. not turbulent) flow. Running the wires through the conduit complicates their replacement. The configuration is also prone to initiating a stationary wave at the lowermost and innermost edge of the distribution body, as shown in FIG. 2.
U.S. Pat. No. 5,382,292 teaches supplying dual wire edge guides from small channels open to the atmosphere rather than from a conduit as shown in FIG. 1f. The channels provide laminar flow and allow time for the action of any surfactant in the lubricating liquid. The wires are more readily accessible for replacement. However, this distribution body may still initiate a stationary wave at the lowermost and innermost edge of the distribution body, as shown in FIG. 2.